An absorbent core constituted by hydrophilic tissue (e.g., pulp) and a water absorbing resin has been in widespread use in sanitary materials such as disposable diapers, sanitary napkins and so-called incontinence pads. The absorbent core is used for the purpose of absorbing bodily fluids.
In recent years, such sanitary materials have been more functionalized and made thinner. The amount of a water absorbing resin used in each sanitary product shows an increasing trend, and the ratio of the water absorbing resin to the entire absorbent core constituted by the water absorbing resin and hydrophilic tissue shows an increasing trend. That is, the sanitary products have been reduced in thickness without a reduction in their absorbing capacity by increasing the ratio of the water absorbing resin in the absorbent core by (i) reducing the amount of hydrophilic tissue which is small in bulk density while (ii) increasing the amount of a water absorbing resin which is excellent in water absorbing property and is large in bulk density.
However, in order to produce sanitary products etc. such as disposable diapers which are to be highly-functionalized and reduced in thickness, it is necessary to incorporate a highly moisture-absorptive water absorbing resin into hydrophilic tissue. Depending on operating environment and climate conditions, the water absorbing resin may cause blocking in a storage hopper or in a transport line or may adhere to an apparatus or a pipe etc. This hinders stable production.
In view of this, there have been disclosed techniques of adding an inorganic substance such as amorphous silicon dioxide or kaoline etc. to a water absorbing resin to obtain a water absorbing agent that is excellent in fluidity after moisture absorption (Patent Literatures 1 to 4). However, the techniques of Patent Literatures 1 to 4 have a problem in which water absorbency against pressure of the water absorbing agent is reduced.
Further, there have been disclosed techniques of coating a surface of a water absorbing resin with polysiloxane or a specific surfactant etc. (Patent Literatures 5 to 8). However, the techniques of Patent Literatures 5 to 8 also have a problem in which, since the water absorbing resin is caused to be excessively hydrophobic or its surface tension is reduced, Re-Wet in disposable diapers is increased. Further, water absorbing resins obtained in Patent Literatures 1 to 15 have, in addition to the problem of fluidity (blocking resistance) after moisture absorption, a problem in which its moisture content is generally small and its particles are low in stability to shock.
Further, according to the technique described in Patent Literature 9, there has been a problem in which a rate of water absorption of a particulate water absorbing agent decreases because a metallic soap is hydrophobic. Therefore, an absorbing article using such a particulate water absorbing agent cannot have sufficient water absorbing property. Further, since the metallic soap is handled in a powdery state, there have been safety and health problems such as deterioration in operating environments due to powder dust or a high risk of dust explosion. Moreover, the techniques disclosed in Patent Literatures 11 and 12 are not sufficient to obtain a recent particulate water absorbing agent having excellent physical properties (e.g., high absorbency against pressure), and the technique described in Patent Literature 15 has a problem in which the yield and productivity are low although particle size is controlled.
In view of such circumstances, there have been disclosed techniques of adding, in order to improve stability to shock and prevent powder dust etc., a small amount of (several wt % of) water to a surface-crosslinked water absorbing resin (Patent Literatures 16 to 20). However, such improved techniques also have a problem in which (i) adding water causes adhesive force between particles and thus applies a heavy load on a mixing apparatus, and as a result, the mixing apparatus tends to stop more frequently and (ii) adding a mixing auxiliary agent (e.g., inorganic salt) causes a reduction in physical properties (e.g., absorbency against pressure) of the water absorbing resin.
That is, the “stability to shock” and “water absorbing property (CRC, AAP) and fluidity after moisture absorption” are in a trade-off relationship. According to the conventional techniques, it has been extremely difficult to improve both of these properties.
The water absorbing agent or the water absorbing resin as has been described is desired to have for example the following properties: excellent free swelling capacity (CRC) and excellent absorbency against pressure (AAP), high liquid permeability (SFC and GBP etc.), high rate of water absorption (FSR and Vortex), high gel strength, and low extractable content (Extr). However, since the water absorbing properties depend on crosslink density, these properties may not be directly proportional to each other. For example, if the crosslink density is increased, the gel strength increases but the water absorbing capacity decreases. In order to suppress such phenomena and to obtain a water absorbing agent that has high absorption capacity and relatively high rate of absorption etc., there has been a method of coating surfaces of water absorbing resin particles with a surfactant or nonvolatile carbon hydride. This method improves dispensability of water absorbed in the initial stage; however, is not effective enough to improve the rate of absorption and suction of each of the particles.
In view of such circumstances, there has been proposed a method of imparting a crosslinked structure not only to inside a water absorbing resin but also to the surface of the water absorbing resin (i.e., surface crosslinking of a water absorbing resin). For example, there have been disclosed methods of increasing crosslink density on a surface of a water absorbing resin by using an organic surface crosslinking agent or an inorganic surface crosslinking agent (Patent Literatures 21 to 23).
Examples of a known crosslinking agent for use in such methods include: polyhydric alcohols; oxazolidinone; alkylene carbonate; polyhydric glycidyl ethers; haloepoxy compounds; polyhydric aldehydes; polyhydric amines; and polyvalent metal salts. However, using such surface crosslinking agents causes a problem in which for example a crosslinkage-forming reaction requires high temperature or takes a long time and some crosslinking agents will remain unreacted.
In view of circumstances, there has been also known a method of surface treatment utilizing a polyvalent metal ionic crosslinking reaction, which can occur at low temperature or room temperature (Patent Literature 23). However, such a method has a problem (i) in which a surface crosslinking agent made from a polyvalent metal generally has low physical properties such as absorbency against pressure (AAP) and (ii) the combined use of a polyvalent metal and some other surface treatment method causes a reduction in absorbency against pressure (AAP). Further, a surface crosslinking reaction, such as a dehydration reaction using a polyhydric alcohol serving as a surface crosslinking agent, requires heat treatment. This excessively reduces moisture content of the water absorbing resin and/or causes coloration of the water absorbing resin. Moreover, there has been a problem in which the reduction in moisture content due to surface crosslinkage reduces powder property (shock resistance).
In view of such circumstances, as an alternative to the surface crosslinkage using the surface crosslinking agent such as those described in Patent Literatures 21 to 23 etc., there have been proposed methods of surface crosslinking using a radical polymerization initiator (Patent Literatures 24, 29, 30) and methods of surface crosslinking for polymerizing monomers on a surface of a water absorbing resin (Patent Literatures 25 to 28). Specifically, for example, there has been known a method of bringing an aqueous solution containing a peroxide radical initiator into contact with resin and heating the resin to decompose the radical initiator, thereby introducing crosslinkage into polymer chains in a shallow surface of the resin (Patent Literature 24).
Further, there has been a method of impregnating a water absorbing resin with water-soluble unsaturated ethylene monomers and polymerizing the water-soluble unsaturated ethylene monomers, and heating the water absorbing resin to obtain a water absorbing resin improved so that the crosslink density in shallow surfaces of water absorbing resin particles is higher than that inside the particles (Patent Literature 25). Furthermore, there have been proposed surface treatment methods each of which is for use in a method of polymerizing monomers on a surface of a water absorbing resin. Each of the surface treatment methods includes adding a radical polymerizable compound to a water absorbing resin and thereafter irradiating the water absorbing resin with activating light (preferably with a ultraviolet ray) (Patent Literatures 26 to 28). Furthermore, there have been proposed surface treatment methods each of which is for use in a method of surface-crosslinking a water absorbing resin with a radical polymerization initiator. Each of the surface treatment methods includes irradiation with activating light (preferably with a ultraviolet ray) (Patent Literatures 29 and 30). Moreover, there has been proposed a surface treatment method using an ultraviolet ray, which method uses a transition metal such as Ag, Fe, Cr and/or Ce etc. (Patent Literature 31)
According to the methods of Patent Literatures 23 to 31, crosslinking can be achieved at low temperatures or room temperature, and a resulting surface-crosslinked water absorbing resin has high moisture content. However, these methods have a problem in which, generally, the absorbency against pressure (AAP), in particular absorbency against pressure under heavy load (AAP 0.7), is difficult to increase.
Further, the surface crosslinking methods described in Patent Literatures 21 to 30 have a problem in which a surface-crosslinked water absorbing resin generally has a low moisture content, and particles of such a water absorbing resin have low stability to shock etc. In order to improve the stability to shock and to prevent powder dust etc., there have been proposed techniques of adding about several percentage of water to a surface-crosslinked water absorbing resin (Patent Literatures 16, 17 and 20). However, such techniques have a problem in which adding water and its auxiliary agent (e.g., inorganic salt) reduces physical properties (e.g., absorbency against pressure) of the water absorbing resin. Further, there has been a problem in which the amount of residual monomers increases due to surface crosslinking (Patent Literature 32).
Further, conventionally, there have been proposed water absorbing resins mainly for disposable diapers, in which water absorbing resin many physical properties such as absorbency against pressure (AAP), water absorbency (CRC) and liquid permeability (GBP, SFC) are controlled (Patent Literatures 17, 20 and 33 to 43). Further, there have been known water absorbing resins each having a high moisture content, such as those described in Patent Literatures 17, 20 and 32. None of these water absorbing resins provide good performances, because a large amount of Re-Wet or leakage etc. occurs when these water absorbing resins are used in disposable diapers.